In recent years, high-temperature superconducting tape materials, which are dramatically improved in property and lengthened, have been produced on a significantly larger scale. Therefore, application of the materials to various types of apparatus has been expected. For example, great advantages may be expected in applying a superconducting coil using a high-temperature superconducting tape material to an apparatus, such as a magnet for silicon monocrystal pulling or a magnetically suspended vehicle. Further, as cooling means for keeping a superconducting coil in the superconducting state, a system of storing the superconducting coil in a vacuum container and conductively cooling the superconducting coil by a cryogenic refrigerator begins to be adopted instead of a system of immersing the superconducting coil in a cryogenic coolant such as liquid helium.
In a case of using an oxide superconducting coil, which has a relatively high critical temperature, as well as a case of using a metal-based superconducting coil, a shield plate is used to block radiation from a vacuum container. As disclosed in Japanese Patents Nos. 2756551, 4095742 and 3486868, a space between a shield plate and a superconducting coil is a vacuum. To reduce a thermal load to the superconducting coil, the superconducting coil must be fixed by a coil supporting member, which has a delicate structure having a small cross-sectional area and which is passed through the shield plate. In addition, a thermal anchor must be provided at the position of the shield plate. Therefore, the mechanical strength of the magnetic apparatus may be low. Furthermore, since the coil supporting member has a delicate structure, production of the member is time-consuming and costly.
Japanese Patent No. 3082397 discloses an apparatus which conductively cools a high-temperature superconducting coil by a cryogenic refrigerator. In this apparatus, to reduce thermal radiation from a room temperature or entry of heat from a current lead, it is necessary to provide a shield plate and a two-stage cryogenic refrigerator and to cool the shield plate in the first stage of the cryogenic refrigerator. Although the superconducting coil uses a high-temperature superconducting material, the problem described above cannot be avoided.
The superconducting magnetic apparatus of the system of conductively cooling the superconducting coil by the cryogenic refrigerator described above generally uses a two-stage cryogenic refrigerator which achieves a temperature of about 4K or lower. However, the two-stage cryogenic refrigerator has a problem with reliability in long-term operation since the refrigerating capacity is considerably influenced by a change in the thermal load. Moreover, since the delicate supporting structure is used to reduce the thermal load in a cryogenic portion, the mechanical strength of the apparatus is low and much time and expenses are required to produce the apparatus. Alternatively, a single-stage cryogenic refrigerator may be used to cool the superconducting coil. The single-stage cryogenic refrigerator achieves a temperature of only about 20K but has a high refrigerating capacity and brings efficiency several times as high as that in the two-stage cryogenic refrigerator. In the case of using the single-stage cryogenic refrigerator, however, the thermal load on the superconducting coil may be increased by the thermal conduction or radiation. As a result, the temperature variation may occur inside the coil.
Under those circumstances, there is a demand for providing a superconducting magnetic apparatus with a simple supporting structure, which has high reliability and realizes cost reduction.